The present invention relates to a power amplifying apparatus which amplifies a plurality of signals through use of a plurality of amplifiers and is of great utility when employed in, for example, a transmitter of a communication satellite using a plurality of electromagnetic wave beams.
A satellite communication system permits the establishment of circuits, or communication channels in wide service areas, as required, and hence allows effective use of a small number of circuits by a large number of earth stations. Conventional satellite communication systems employ a single beam for transmission and reception, but a multi-beam satellite communication system employing a plurality of beams is now being proposed as a next-generation satellite communication system. This system covers a plurality of service areas 11.sub.1, 11.sub.2, 11.sub.3, . . . on the earth by antenna beams 10.sub.1, 10.sub.2, 10.sub.3, . . . from a communication satellite 100, respectively, as shown in FIG. 1. With this multi-beam system, it is possible to increase the satellite antenna gain and to use the same frequency, and accordingly an increase in the transmission capacity or miniaturization of an earth station can be expected.
In such a multi-beam system, a power amplifying apparatus for supplying transmitting power to transmitting antennas 9.sub.1, 9.sub.2, 9.sub.3, . . . for the beams 10.sub.1, 10.sub.2, 10.sub.3, . . . has such an arrangement as depicted in FIG. 1. Input terminals 1.sub.1 to 1.sub.n are respectively connected to input sides of power amplifiers (hereinafter referred to simply as amplifiers) 2.sub.1 and to 2.sub.n each of a maximum transmission output P. The output sides of the amplifiers 2.sub.1 to 2.sub.n are connected to output terminals 3.sub.1 to 3.sub.n, respectively. Input signals to the input terminals 1.sub.1 to 1.sub.n are individually amplified by the amplifiers 2.sub.1 to 2.sub.n and delivered to the output terminals 3.sub.1 to 3.sub.n.
In this amplifying apparatus, the transmission output at each output terminal is limited by the output P of each of the amplifiers 2.sub.1 to 2.sub.n. That is to say, the path between each input terminal and the corresponding output terminal is constructed completely independently of the other paths. On account of this, for instance, even if the amplifier 2.sub.1 has a margin in its power amplification capacity for the signal which is applied thereto from the input terminal 1.sub.1 and amplified thereby for output to the output terminal 3.sub.1, the margin cannot be used for signals which are provided to the other output terminals.
In multi-beam satellite communication, the number of carriers which each beam transmits varies with the amount of communication traffic in each of the service areas 11.sub.1, 11.sub.2, 11.sub.3, . . . ; therefore, it is necessary that each of the amplifiers 2.sub.1, 2.sub.2, . . . for the beams 10.sub.1, 10.sub.2, . . . have a power amplification capacity large enough to sufficiently amplify the input signal when it is assigned the largest number of carriers. To meet this requirement, an expensive amplifier of a large power capacity must be prepared for each beam, but when the number of carriers assigned to the input signal is small, such capacity of the amplifier is not effectively utilized. Furthermore, even in the case where the amplifier has a margin in its output but another amplifier wants more output, the surplus power cannot be assigned to the beam of insufficient power.
In view of the above, there has been proposed by W. A. Sandrin a system in which an input signal for each beam is equally distributed to a plurality of amplifiers, and then the amplified signals are combined into signals for the respective beams for output as the corresponding antenna beams ("The Butler matrix transponder", COMSAT Tech. Review, Vol. 4, No. 2, pp. 319-345, Fall 1974). In this system, a Butler matrix circuit is disposed between a plurality of input terminals corresponding to the respective beams and the input sides of a plurality of amplifiers and another Butler matrix circuit is disposed between the output sides of the amplifiers and feeding points for the respective beams. The Butler matrix circuits on the input and output sides are each made up of 90.degree. hybrid couplers and fixed phase shifters. A required amount of phase shift by each fixed phase shifter varies with the number of amplifiers used, but when four amplifiers are used, the phase shift is an integral multiple of 45.degree.. Such a phase shifter is formed by a transmission line and the phase shift is dependent upon the length of the transmission line. Therefore, a desired phase shift is obtainable at only one frequency, so a wide-band characteristic cannot be obtained. Moreover, an increase in the number of amplifiers used causes an increase in the number of fixed phase shifters, and the above system is difficult to implement when the number of amplifiers used is as many as 16 or more.
Furthermore, what is called a balanced amplifier has been proposed with a view to effectively utilizing amplifiers in two signal systems. As illustrated in FIG. 2, the input terminals 1.sub.1 and 1.sub.2 are connected to two input terminals of a 90.degree. hybrid coupler 121, respectively, which has its two output terminals connected via the amplifiers 2.sub.1 and 2.sub.2 to two input terminals of another 90.degree. hybrid coupler 122, the two output terminals of which are, in turn, connected to the output terminals 3.sub.1 and 3.sub.2. Letting the amplification gain of each of the amplifiers 2.sub.1 and 2.sub.2 be represented by a, an input signal P.sub.1 to the input terminal 1.sub.1 is divided by the hybrid coupler 121 into two equal signals, which are respectively amplified by the amplifiers 2.sub.1 and 2.sub.2, and the amplified outputs are combined by the hybrid coupler 122 into a composite signal, which is delivered as a.P.sub.1 at the output terminal 3.sub.2, but no output is obtained at the output terminal 3.sub.1. Similarly, a signal P.sub.2 input to the input terminal 1.sub.2 is amplified to a.P.sub.2 and output to the output terminal 3.sub.1 but is not delivered to the output terminal 3.sub.2.
In this balanced amplifier, when differences in the gain and the phase shift arise between the amplifiers 2.sub.1 and 2.sub.2, the component of the signal P.sub.1 will appear at the output terminal 3.sub.1, degrading isolation (crosstalk). Besides, even a slight difference in either the gain or phase shift between the amplifiers 2.sub.1 and 2.sub.2 will greatly deteriorate the isolation, and a failure of one of them will lead to marked reduction of the output and degradation of the isolation, making the balanced amplifier inoperable.